Malcolm K. Stanford and Walter A. Wozniak

Glenn Research Center, Cleveland, Ohio

Terry R. McCue

ASRC Aerospace Corporation, Cleveland, Ohio 

Addressing Machining Issues for the 

Intermetallic Compound 60-NITINOL

NASA/TM—2012-216027

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Malcolm K. Stanford and Walter A. Wozniak

Glenn Research Center, Cleveland, Ohio

Terry R. McCue

ASRC Aerospace Corporation, Cleveland, Ohio 

Addressing Machining Issues for the 

Intermetallic Compound 60-NITINOL

NASA/TM—2012-216027

August 2012


	



	


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NASA/TM—2012-216027 1 

Addressing Machining Issues for the 

Intermetallic Compound 60-NITINOL 

Malcolm K. Stanford and Walter A. Wozniak 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Terry R. McCue 

ASRC Aerospace Corporation 

Cleveland, Ohio 44135 

Abstract 

60-NITINOL (60 wt.% Ni – 40 wt.% Ti) is being studied as a material for advanced aerospace 

components. Frequent wire breakage during electrical-discharge machining of this material was 

investigated. The studied material was fabricated from hot isostatically pressed 60-NITINOL powder 

obtained through a commercial source. Bulk chemical analysis of the material showed that the 

composition was nominal but had relatively high levels of certain impurities, including Al and O. It was 

later determined that Al2O3 particles had contaminated the material during the hot isostatic pressing 

procedure and that these particles were the most likely cause of the wire breakage. The results of this 

investigation highlight the importance of material cleanliness to its further implementation. 

Introduction 

Recent research and development at NASA Glenn Research Center in conjunction with industrial 

partners has identified a combination of properties possessed by 60-NITINOL that make this material an 

excellent candidate for advanced bearings and gears (Refs. 1 and 2). Like its well-known cousin, 

55-NITINOL (Ref. 3), this intermetallic material also displays the shape memory effect. Among other 

properties, 60-NITINOL has excellent aqueous corrosion resistance, is non-magnetic and has a modulus 

nearly half that of typical steels used in bearing and gear applications. 

60-NITINOL is an ordered intermetallic with the CsCl-type B2 crystal structure. In this arrangement, 

atoms of one type (i.e., Ni) are at the eight corners of a cubic unit cell and an atom of another type (i.e., 

Ti) sits at the center of the cube, as depicted schematically in Figure 1. Though composed of metallic 

atoms, the high degree of atomic ordering in this material causes it to behave more like a covalently 

bonded ceramic with intrinsic brittleness. However, this atomic structure also enables precipitation 

hardening that results in hardness equivalent to that of conventional bearing materials. This collection of 

unique properties gives this material a myriad of potential aerospace applications never before explored 

with intermetallic materials. 

During the preparation of prototype 60-NITINOL test components, electrical-discharge machining 

wires broke repeatedly leading to a great deal of machining downtime. This paper discusses how this 

issue was addressed. 
NASA/TM—2012-216027 2 

Materials and Procedures 

Gas atomized intermetallic 60-NITINOL powder was obtained from a commercial source. The 

powder was -60 mesh, as classified by screening. A sample of the powder was examined by field-effect 

scanning electron microscopy (SEM) and by optical microscopy. The chemical composition of the 

powder was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The 

crystalline phases present in the powder were identified by x-ray diffraction (XRD) using Cu K

radiation. The apparent density and the tapped density were measured using standard test methods 

(Refs. 4 and 5). The gravity-driven flow rate of the powder was measured using a Hall flowmeter 

(Ref. 6). Particle size analysis was performed using the light scattering technique (Ref. 7). 

The powder was consolidated into a compact by hot isostatic pressing (HIPping) as depicted in 

Figure 2. After HIPping, the cylindrical stainless steel container was machined away from the compact 

and the material was sectioned diametrally into slices approximately 6 mm thick. Blanks for inner and 

outer bearing races were subsequently sectioned from these slices. All of the machining was done by wire 

electrical-discharge machining (EDM). 

The EDM wire broke several times during the machining of this material. Careful observation after 

one occurrence revealed a small irregularity in the material where the wire broke. Therefore, this 

irregularity was suspected to have a role in the EDM wire breakage. Remnants of the material were 

(a) (b) 

Figure 1.—60-NITINOL B2 unit cell showing (a) atomic arrangement and (b) relative size of 

atoms in the unit cell (Image generated with Materials Studio modeling package from 

Accelrys Software, Inc.). 

Figure 2.—Schematic description of the steps involved in the hot isostatic pressing (HIPping) process 

(Ref. 8). 

Ti 

Ni 
NASA/TM—2012-216027 3 

sectioned and prepared for metallographic examination and microindentation hardness testing. Chemical 

analysis of the bulk material was performed by ICP-OES. Phase identification was performed by energydispersive x-ray spectroscopy (EDS) and by XRD. 

Results and Discussion 

Scanning electron photomicrographs of the 60-NITINOL powder are shown in Figure 3. The 

generally spherical shape of the powder is typical of gas atomized powders. Some larger particles have 

satellites from where smaller particles impacted with them before they were completely solidified (see 

Figure 3(b)). There is also a small portion of irregularly shaped particles within the powder, which is not 

abnormal with this powder fabrication technique, though these particles are typically removed during the 

screening step. Images of powder particle cross-sections are shown in Figure 4. The remaining powder 

characterization data is listed in Table I. One important item to note is the relatively high level of oxygen, 

iron, aluminum and copper impurities. 

(a) 

(b) 

Figure 3.—Scanning electron photomicrograph of the 60-NITINOL powder 

characterized in Table I shown at (a) 200X original magnification and (b) 

2,000X original magnification. 
NASA/TM—2012-216027 4 

(a)

(b)

Figure 4.—(a) Optical photomicrographs of cross-sections of the 60-NITINOL characterized in Table I 

shown and (b) a higher-magnification image. The polished specimen was swab-etched with a room 

temperature solution composed of 1HF+4HNO3+5H2O. 
NASA/TM—2012-216027 5 

TABLE I.—CHARACTERIZATION OF 60-NITINOL POWDER SHOWN IN FIGURE 3 AND FIGURE 4 

Composition 59.7wt%Ni-40.1wt%Ti 

Impurities (ppm) O (655), Fe (480), Al (230), Cu (45), Cr (30), Co (20), V (20) 

Crystalline phases 47% Cubic (B2) NiTi 

53% Rhombohedral (D024) Ni3Ti

Apparent density 4.0 ± 0.0 g/cm3 (70%*) 

Tap density 4.9 ± 0.0 g/cm3 (86%*) 

Hall flow time (50g sample) 16.6 ± 0.4 sec 

Particle size Dmean = 100.5 ± 60.7 μm 

D5 = 21.5 μm 

D10 = 30.7 μm 

D50 = 94.0 μm 

D90 = 178.7 μm 

D95 = 201.7 μm 

*The density is expressed as a percentage of the theoretical density of B2 NiTi (6.556 g/cm3), based on 

x-ray powder diffraction data. 

Figure 5.—Optical photomicrograph of 60-NITINOL compact after hot isostatic pressing. The polished 

specimen was swab-etched for 20 seconds with a room temperature aqueous solution composed of 10 

vol.% HF and 5 vol.% HNO3. 

XRD determined that the two phases present in the compact are cubic NiTi and orthorhombic Ni3Ti. 

The microstructure of the as-received HIPped 60-NITINOL material is shown in Figure 5. EDS was used 

to confirm that the parent phase is NiTi and the light grey second phase dispersed throughout the 

microstructure is Ni3Ti. The grain boundaries represent the prior particle boundaries. An SEM 

photomicrograph and an elemental dot mapping of the 60-NITINOL compact near the stainless steel 

HIPping can are shown in Figure 6. The can is visible along the left of the image. These images merely 
NASA/TM—2012-216027 6 

help to confirm that the composition of the material is uniform within the compact. Additional compact 

characterization data, including the bulk chemical composition, is listed in Table II. Again, the levels of 

oxygen, iron, aluminum and copper (detected at levels on the order of hundreds of parts per million) 

should be noted. 

(a) 

 Ni Fe 

(b) (d) 

 Ti C 

(c) (e) 

Figure 6.—(a) SEM photomicrograph of the 60-NITINOL compact near the 

stainless steel container (seen on the left of image) and (b) to (e) x-ray 

elemental dot maps from this image showing the major elements present. 

TABLE II.—PHYSICAL PROPERTIES OF THE AS-RECEIVED 60-NITINOL COMPACT 

Composition 59.9wt%Ni- 39.9wt%Ti 

Impurities (ppm) O (690), Fe (490), Al (210), Cu (105), Cr (30), Co (20), V (20) 

Density 6.645 g/cm3

Hardness 339.2 ± 9.0 HV500
NASA/TM—2012-216027 7 

Figure 7.—SEM photomicrograph of an inclusion discovered on the inner diameter 

of an outer bearing race blank. 

Upon closer examination of the irregularity suspected to be involved in an EDM wire breakage, it was 

identified as a particle embedded in the material (see Figure 7). As shown in Figure 8, the particle was 

composed of aluminum and oxygen. The ratio of these two elements indicates that the particle is 

aluminum oxide (Al2O3). The particle was also splattered with a small amount of a copper alloy (brass), 

which was transferred from the EDM wire (see Figure 9). 

It was later discovered that an abrasive flap wheel had been used during the procedure used to prepare 

the HIP container. A sample of the flap wheel abrasive was obtained. The abrasive particles were found to 

be embedded in an adhesive material and then covered with a coating that allows the abrasive to be 

exposed gradually during its intended use. The coating was dissolved away with acetone to facilitate SEM 

analysis of the abrasive particles. Figure 10 shows the abrasive particles partially exposed through the 

coating. These particles are on the order of hundreds of microns in size and develop static charge when 

probed with the SEM electron beam. A typical x-ray spectrum of one of these particles (Figure 10(b)) 

indicates that they are indeed the same composition as the particle embedded in the 60-NITINOL 

(namely, Al2O3). This partially accounts for impurities detected in bulk chemical analyses (i.e., Al and O). 

Clearly, the EDM wire breakage was caused in this case because the wire was unable to pass through this 

electrically insulating inclusion. This was most likely the case with most (if not all) of the other wire 

breakages. 

This finding provides an opportunity to highlight an important issue. Due to the brittle nature of 

intermetallics, material cleanliness is extremely important. It is especially important to prevent ceramic 

inclusions because they tend to be hard and faceted, which makes them excellent crack initiators. Any 

tensile stress near such an inclusion could lead to a failure. This issue is discussed in a recent publication 

that addresses quench cracking of 60-NITINOL due to ZrO2 contamination (Ref. 9). The alumina 

contamination studied here was easily eliminated by modifying the HIP container preparation procedure. 
NASA/TM—2012-216027 8 

Figure 8.—SEM photomicrograph of the embedded particle. The first x-ray spectrum 

(EDS-A) indicates that the particle is composed of aluminum and oxygen in a ratio that 

follows the weight percentage of these elements in alumina. The second spectrum 

(EDS-B) confirms that the parent phase is 60-NITINOL. 
NASA/TM—2012-216027 9 

Figure 9.—SEM photomicrograph of surface of embedded particle shown in Figure 8 with 

EDS spectra indicating a composition of alumina (EDS-C) splatter with brass from the 

EDM wire (EDS-D). 
NASA/TM—2012-216027 10 

Figure 10.—(a) SEM photomicrograph of surface of flap wheel abrasive paper showing 

abrasive particles partially exposed and (b) the EDS spectrum of one such particle as 

indicated in the figure. 
NASA/TM—2012-216027 11 

Conclusions 

The cause of frequent EDM wire breakage during the machining of HIPped 60-NITINOL has been 

investigated. Based on the results of this investigation, it can be concluded that Al2O3 inclusions caused a 

number of EDM wire breakages during the machining of this material. More generally, any type of 

ceramic inclusion would tend to degrade material properties such as machinability and structural integrity. 

References 

1. C. DellaCorte, S.V. Pepper, R. Noebe, D.R. Hull, G. Glennon, “Intermetallic Nickel-Titanium Alloys 

for Oil-Lubricated Bearing Applications,” NASA/TM—2009-215646, March 2009, National 

Technical Information Service, Springfield, VA. 

2. C. DellaCorte, R.D. Noebe, M.K. Stanford, S.A. Padula, “Resilient and Corrosion-Proof Rolling 

Element Bearings Made From Superelastic Ni-Ti Alloys for Aerospace Mechanism Applications,” 

NASA/TM—2011-217105, September 2011, National Technical Information Service, Springfield, 

VA. 

3. W.J. Buehler, “Intermetallic Compound Based Materials for Structural Applications,” in The Seventh 

Navy Science Symposium: Solution to Navy Problems through Advanced Technology, May 14, 15, 16, 

1963, U.S. Naval Aviation Medical Center, Pensacola, Florida. Vol. 1, Office of Naval Research, 

Arlington, VA, 16 May 1963. 

4. ASTM Designation B212-09, “Standard Test Method for Apparent Density of Free-Flowing Metal 

Powders Using the Hall Flowmeter Funnel,” 2011, vol. 02.05, American Society for Testing and 

Materials, West Conshokocken, PA. 

5. ASTM Designation B527-06, “Standard Test Method for Determination of Tap Density of Metallic 

Powders and Compounds,” 2011, vol. 02.05, American Society for Testing and Materials, West 

Conshokocken, PA. 

6. ASTM Designation B213, “Standard Test Method for Flow Rate of Metal Powders,” 2011, vol. 

02.05, American Society for Testing and Materials, West Conshokocken, PA. 

7. ASTM Designation B822 – 10, “Standard Test Method for Particle Size Distribution of Metal 

Powders and Related Compounds by Light Scattering,” 2011, vol. 02.05, American Society for 

Testing and Materials, West Conshokocken, PA. 

8. R.M. German, Powder Metallurgy & Particulate Materials Processing, 2005, Metal Powder 

Industries Federation, Princeton, NJ. 

9. M.K. Stanford, F. Thomas and C. DellaCorte, “Processing Issues for Preliminary Melts of the 

Intermetallic Compound 60-NITINOL,” in preparation as a NASA Technical Memorandum, National 

Technical Information Service, Springfield, VA.